OvmfPkg/PciHostBridgeLib: remove Bhyve and Xen support

OvmfPkg/PciHostBridgeLib: remove Bhyve and Xen support

The "OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.inf" instance is
used by the following platforms in edk2:

  OvmfPkg/AmdSev/AmdSevX64.dsc
  OvmfPkg/OvmfPkgIa32.dsc
  OvmfPkg/OvmfPkgIa32X64.dsc
  OvmfPkg/OvmfPkgX64.dsc

All these platforms statically inherit PcdPciDisableBusEnumeration=FALSE
from "MdeModulePkg.dec". Remove the the PCD and everything that depends on
it from the PciHostBridgeLib instance. Namely, remove the logic that
determines the root bridge apertures by (a) scanning the entire bus,
device and function number space, and (b) parsing the BAR values that were
pre-set by the Bhyve or Xen machinery.

"XenSupport.c" used to be listed explicitly in "Maintainers.txt", remove
it from that spot too.

Cc: Andrew Fish <afish@apple.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210526201446.12554-33-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>